The present invention relates to a control circuit for a half wave brushless motor.
Each time the electronic power switch (typically a MOSFET transistor) associated with a winding or phase of the motor is disabled, a high over voltage is created in this winding due to the energy inductively stored in this winding during the preceding period when the power switch is turned on.
To limit this voltage surge it is known to utilise, for example, Zener diodes disposed between the output electrode and the input electrode of the power switch (or between the output electrode and earth). This arrangement allows the voltage surge to be limited to a fixed threshold value.
This arrangement is not entirely satisfactory.
In fact, the amplitude of the voltage or back electromotive force developed in each winding or phase of the motor varies with variation of the speed of rotation thereof. This amplitude is particularly modest at low speeds, and higher at high speed. The fixed voltage threshold to which the voltage surge is limited with the known arrangements is however established on the basis of the value which this voltage surge assumes at high speed. Consequently at low speed, and therefore with relatively low back electromotive force values, rather high surge voltage peaks are allowed relative to the average amplitude of the back electromotive force. This leads to the generation of an unpleasant acoustic noise in operation.